At the beginning of the Jim Carrey movie, Liar, Liar, in order to demonstrate how much of a liar Carrey's character is, they show him promise to show up for his son's birthday party and then break that promise. The son didn't believe him when he said he would be there so he made a promise in order to demonstrate that this time he would keep his word. The son still didn't believe him but still hoped and had his hopes crushed which, of course, led to the magical birthday wish that prevented Carrey from being able to lie for the rest of the movie. Carrey's promise was the deperate tactic of a man whose word wasn't any good but wanted to be believed. In the end, though, making a promise only made him more pathetic. After all, breaking your word is one thing but breaking a promise is worse, right?
James, the brother of Jesus, addressed this dynamic in James 5:12, saying, 'Now above all, my brothers, do not swear, either by heaven or by earth or with any other oath. Your “yes” must be “yes,” and your “no” must be “no,” so that you won’t fall under judgment' (HCSB). James' response to the entire issue was to never swear, to never make a promise. In part, this mindset was an extension of the thought from chapter 4 where James warned people not to say definitively what the future held because they couldn't know what God would do. And if you can't know what the future holds, making a promise is essentially setting yourself up to break your word.
Instead of making promises to let people know how serious your are or so that people will know that you mean it this time, speak in such a way that your word is good all the time. Let your "yes" be "yes" and your "no" be "no." Say exactly what you mean and don't oversell yourself. If you might not be able to do something, say that. If something is possible but not certain, say that. If someone asks a favor but you have more important priorities then be honest. Such honesty will strip away the disguises we build, making us more transparent and genuine (maybe even motivating us to change our priorities), and will cause people to believe us, even when we don't make promises.
Don't make promises so that people will believe you really mean it. Speak in such a way that your word is always good and there is no need for promises.